Heaters for heating water in swimming pools are well known in the prior art. The majority of pool heaters presently in use are gas fired. In such devices, the hot products of combustion are passed through a heat exchanger. Water from the pool is also passed through the heat exchanger and absorbs heat from the products of combustion. While such gas fired units are reliable, they are inefficient. The best theoretical coefficient of performance for such a system is 1:1. Of course the coefficient of performance will always be somewhat less due to losses. This makes a conventional gas fired pool heater expensive to operate.
Other types of pool heaters known in the prior art are electrically powered heat pumps. Such systems use a working fluid such as Freon 22 or other refrigerant, to absorb heat from the atmosphere in an evaporator, resulting in vaporization of the working fluid. The working fluid is then compressed in a compressor and passed to a heat exchanger or condenser that is in heat transfer relation with the pool water. In the heat exchanger the working fluid delivers heat to the pool water and is condensed to a liquid. Thereafter the liquid working fluid flows through an expansion device and returns to the evaporator to complete the cycle. The working fluid continuously flows in the heat pump system to deliver heat from the atmosphere to the pool water.
Because a heat pump system uses heat available from the atmosphere to heat the pool water, such systems may have coefficients of performance in the range of 4:1. However electric heat pump systems may be more expensive to operate than gas fired systems because electricity generally costs more than natural gas. Electric heat pump systems also have a disadvantage in that when the ambient temperature is low, the efficiency of the heat pump system falls. As a result, it is usually necessary to have a supplemental heating system such as a gas fired heater or an electrical resistance heater. Electric heat pump systems also characteristically require more maintenance than gas fired systems which adds to their overall cost.
The need to have a supplemental heating system with a heat pump system increases when the pool is heated in combination with a "hot tub" or spa. People enjoy using their spas year round. In colder climates during the winter a heat pump system alone will not satisfactorily heat the spa water.
Thus, there exists a need for a pool and spa heating system that is less expensive to operate than those known in the prior art, has higher efficiency, is more reliable and can be used in cold weather. There further exists a need for a novel power unit that may be used in such a pool heating, or other type of heat transfer system.